The word “cable” also stands for the word “line”. “Cable” can be an electrical or optical cable with any design of the core which is surrounded by a sheath of insulating material. “Transmission elements” can be metallic electrical conductors or optical waveguides. Such cables have a protective outer sheath of insulating material with different characteristics depending on the type of the cable and the field of use of the same. The thickness of the sheath is variable depending on the mentioned characteristics. The amount of material for forming the sheath of such cables normally is high. The portion of the sheath to the weight of the complete cable is considerable.
WO 98/52197 describes a power transmission cable with an outer coating made of expanded polymer material. The coating shall be capable of protecting the cable from accidental impacts. A separate metal armor shall not be needed. The coating therefore has special mechanical characteristics to absorb impacts. The used material has a degree of expansion from 20% to 3000% and a flexural modulus between 400 MPa and 1800 MPa. Such a material is expensive. Its weight is lower compared to the not expanded version. But for the purpose of impact protection the coating of expanded polymer material needs an adequate thickness and flexural modulus together with a great mass. Therefrom the weight of the sheath still is high. Such a coating therefore only is useful with cables which normally have a metal armor and then can be manufactured without such an armor.
WO 98/52197 also mentions documents which describe cables for the transmission of signals with a layer of expanded insulating material. Such a material shall be useful only for the increase of the transmission speed of the signals. A hint for impact protection shall not to be found in these documents.
From GB specification 1 339 561, an electrical cable is known, preferably a telecommunication cable, which also shall be protected against mechanical stresses, like impacts, without a special armoring. The core of the cable is surrounded by a layer of expanded insulating material which is surrounded by a layer of not expanded insulating material. The expanded layer has a greater thickness then the not expanded layer. This known cable is comparable with the cable of WO 98/52197.